1. Field of the Invention
The present invention relates to an inorganic silicon nitride-based fiber and a composite material reinforced by the fiber. The inorganic silicon nitride-based fiber of the present invention has a higher strength and a higher modulus of elasticity than known silicon nitride-based fibers and a high adhesion with various matrixes including metals, organic resins and rubbers, and ceramics, thereby providing a composite material having a high strength, a high modulus of elasticity and a high heat resistance.
2. Description of the Related Art
The silicon nitride has excellent properties including mechanical strength, thermalshock resistance, oxidation resistance, chemical resistance, wettability with metals, and electrical insulation, and is now widely used as industrial heat resistant materials and abrasion resistant materials. Generally, ceramic materials are known to have remarkably increased properties including mechanical strengths when formed into fibers. Therefore, silicon nitride formed into fibers may have the above various advantageous properties and may be expected to be used as heat resistant materials in mesh belts, conveyer belts, curtain, filters etc., and as reinforcements in composite materials such as engine parts, fan blades, aircraft structures etc., by being processed into various forms including woven fabrics, felts, ropes, yarns, and chopped strands.
Various processes have been proposed for the production of silicon nitride fibers. Some of the more important processes are listed below:
(1) a process, wherein silicon monoxide (SiO) prepared by reducing silicate at a high temperature is reacted with ammonia and hydrogen at 1425.degree. to 1455.degree. C. to form fibrous silicon nitride, 5 to 30 .mu.m in diameter and about 370 mm long, on a graphite substrate. (Cunningham et al., 15th Nat. SAMPE Symp., 1969),
(2) a process, wherein an organic polycondensate having one or more silazane (SiN) groups is melt-spun to form a fibrous polymer, which is fired in an ammonia atmosphere (Ishikawa et al., Japanese Patent Laid-Open No. 200210/1982), and
(3) a process, wherein organosilizane fibers prepared by melt spinning or dry spinning organosilazanes are fired in an inert gas atmosphere to produce silicon carbide/silicon nitride composite fibers (G. Winter et al., Japanese Patent Laid-Open No. 69717/1974; W. Verbeek et al., Japanese Patent Laid-Open No. 20206/1974; Penn et al., "J. of Applied Polymer Science", Vol. 27, 3751-3761 (1982); Penn et al., I.E.C., "Proc. Des. Dev.", Vol. 23, No. 2, 217-220 (1984); Seyferth et al., U.S. Pat. No. 4,482,669).
Each of the above processes and silicon nitride fibers produced thereby have specific problems. The process (1) is incapable of making silicon nitride into continuous fibers, and is not suited for mass production because of its poor controllability. The major problems associated with the processes (2) and (3) result from higher carbon contents in their starting fibers, with the result that the final products will have higher concentrations of silicon carbide and/or free carbon. This may sometimes cause formation of cracks, voids and pores during the thermal decomposition step, which will degrade some of the properties inherent in silicon nitride, such as electrical insulation, mechanical strength and resistance to thermal shocks. Furtheremore, the fibers prepared by the processes (2) and (3) are insufficient in tensile strength and electrical resistivity, which are known in the art to have to be in the order of 41 to 142 kg/mm.sup.2 and 7 .times.10.sup.8 .OMEGA..multidot.cm, to be used for aerospace materials.
Composite materials using fibers as a reinforcement are known. For example, metals are reinforced with inorganic fibers such as silica fibers, alumina fibers, or carbon fibers. But silica fiber reinforced metals have a low modulus of elasticity and an insufficiently high strength; alumina fibers do not sufficiently increase in strength and modulus of elasticity etc. due to a low compatibility or wettability with metals; and carbon fibers have a large reactivity with metals, resulting in a lower strength of metals. Metals reinforced with silicon carbide fibers are also known, but the strength of silicon carbide fibers is lowered when immersed in molten metals and when heated after reinforcement due to a reaction with the metals. Furthermore, a conventional reinforcement of metals with inorganic fibers generally requires a surface treatment of the fibers to prevent a lowering of the strength thereof as well as a prolonged heat treatment, which are disadvantageous.
Composite materials of ceramics reinforced with silica fibers, alumina fibers or silicon carbide fibers are also known. The production cost for these fibers is very high, and the silica fibers have a low modulus of elasticity and the alumina fibers have a low thermal shock resistance. The silicon carbide fibers provide an excellent thermal resistance to metals, but it is difficult to obtain uniform silicon carbide fibers, resulting in nonuniform properties of the composite material. Moreover, the cost of silicon carbide fibers is high.
Resine or rubbers are reinforced with whiskers, glass fibers, alamide fibers, alumina fibers, silicon carbide fibers, etc. Whiskers are difficult to arrange in required directions in resins, etc., due to their nonuniform length and diameter, resulting in low strength of the composite materials. Glass fibers, which are most widely used at present, have a low strength and low modulus of elasticity and do not provide higher strength composite materials. Carbon fibers do not have a good wettability with resins and rubbers and are electrically conductive, which limits the application thereof. Alamide fibers have a reduced stiffness and reduced antioxydation in comparison with inorganic fibers. Alumina fibers have a high density and a low strength, although electrically insulative and electromagnetic wave transmissive and having a high modulus of elasticity. Silicon carbide fibers are not suitable for various uses due to the very high electrical insulation, etc., thereof.